The invention relates to an apparatus for the feeding of starting materials into the shearing gap of machines for the production of plastic films, ribbon, foils or plates, in which the shearing gap is bounded, on a long side, by a horizontal, heated, turnable roller.
Machines for the production of films, ribbons, foils or plates of thermoplastic and duroplastic plastic materials, including the elastomer plastics, are adequately well known. As used herein, "duroplastic" refers to a "thermosetting resin." Such machines consist of a roller with a corresponding fixed counterpart or several rollers between which the plasticized plastics are melted, kneaded and homogenized. The end product is generally taken off as an endless foil from the last roller.
There also belong to these machines the so-called coating machines, in which the plastic ribbon is taken from the last roller by means of a carrier ribbon. There the carrier ribbon and plastic foil are inseparably united with one another and form a so-called composite foil.
An essential element of these machines is the portion where the starting materials are fed. In the majority of all the machines, there is used a so-called "shearing gap," the designation of which has come about by the manner in which the parts forming it function. Involved is a wedge-shaped chamber (in the gap) with the narrower section directed downward, whose length corresponds substantially to the width of the ribbon to be generated. One longitudinal wall of the chamber is formed by a turnable heated roller, whose surface moves downward in the zone of the shearing gap and, in the process, carries along a certain amount of molten or softened plastic through the gap opening. The other longitudinal wall can be variously constructed. It is possible, for example, to construct the other wall as a fixed wedge, shoe or also as a bar. As a longitudinal wall, however, there also comes in question a second roller, which turns opposite to the first and with a lower circumferential velocity. Through the differing circumferential velocity between the rollers, there comes about a shearing effect in the plastic composition. The processing in the shearing gap is of determinative importance since it determines the properties of the end product and the economics of the process. The feed-in process for the starting materials is important since the type of feed (in part) influences the processes in the shearing gap.
For some time, the so-called ribbons have had considerable significance. Carrier ribbons which are coated (lined) with plastics yield composite materials which have new and better properties than the two starting materials by themselves. Excellent possibilities of use are yielded in respect to furniture covering materials, curtain materials, protective clothing, decorative materials and wallpapers, patent leather, for example, for seat coverings, tarpaulins and tents, lattice fabrics permeable to light, bags, sealable inlays for material reinforcement, floor coverings, packings, sealable papers, insulating papers for buildings, packings for closures of flanges, book bindings, etc. From this enumeration, it is evident that the process described is utilized for a large number of products which are needed in large quantities. Experience has shown that the demand for the corresponding products has risen recently with more than average sharpness and, in the future, will rise further. It is obvious that with such a demand, improvements in the production process and of the devices required therein will have considerable effect on the pricing, or with a constant price, will permit the production of higher value products. In particular, a production increase in existing installation is an advantage not to be underestimated.
An an apparatus that come under the category described is known; for example, see German Patent No. 1,213,107. Closed fabrics, lattice fabrics, knits, tricot, fleeces, non-woven fabrics of threads or fibers of all kinds, including plastics, papers of every type, windable cardboard, foils of synthetic leather or metal, etc. serve as carrier ribbons. Thermoplastic materials used include hard and soft PVC (polyvinyl chloride), high-pressure and low-pressure polyolefins, polyurethane, polyamides, polystyrene, cellulose acetate, ionomers, acetobutyrate, copolymerizates, foamable polymers and synthetic and natural waxes. Papers and fabrics become, depending on the exact coating material utilized, impermeable to liquids, water vapor, gas, fats and aroma substances. They can be welded or hot-sealed, as desired in packaging technology or in the wrapping (konfektionierung) of textiles. For the packing of foods or pharmaceutical articles, physiologically faultless materials are produced with suitable thermoplasts. Tearing strength or abrasions resistance can be increased. By choice of the plastic color and use of corresponding embossing rollers, there are achieved manifold surface effects. Composite foils combine the advantageous properties of carrier foil and coating materials.
The starting materials are not restricted to thermoplastic materials. Thermosetting resins (duroplasts) can also be used such as, for example, unsaturated polyester resins and elastomers, and synthetic and natural types of rubber such as neoprenes, butadienes, acrylonitriles and natural rubber. In the case of composite materials, there are used in addition to elastic carrier webs, rigid carriers, such as plates or plaques.
There exists a substantial problem in the known apparatus in respect to the charging of the apparatus with plastics. It is a known practice to carry out the charging by feeding in nonplastic materials, such as granulate, powder, dry blend, blocks and pastes. This type of feed is used mainly in the so-called melt roll process wherein the plastic is supplied from a supply container over a controllable dosage device to the roller gap. The scope of use, however, is limited by a restricted output capacity and the second hot process required for the production of the granulate.
There has also been used a hot-feed of the thermoplastic materials over the so-called Banbury-mill route. Here, there is used a type of preplastification, which is used mainly in calendering machines. The thermoplastic material is worked up in a stamp kneader and plasticized and thereupon supplied discontinuously over a conveyor belt to the milling machine. The plasticized material is then cut and rolled on the milling machine and supplied to the first roller mouth. With this method there are associated therewith the disadvantages of a discontinuous process with irregular feed and poor feed form (packages). Due to the latter, there occur different viscosities of the plastic in the roller gap, so that the production of uniform, good end products is rendered difficult if not even prevented.
There also exists an extruder feed carried out by means of a swinging arm. Here, the premixed material is supplied to an extruder and plasticized into a continuously emerging plastic string. This string is supplied over a swinging arm in pendulation to the roller mouth. It is regarded as disadvantageous that the swinging movement of the plastic string causes an uneven charging in width. The charging in width varies, moreover, with the feed velocity. A further restriction on the possibilities of using this method is that only thermoplastic materials in a certain melt viscosity range can be used. There occur, therefore, viscosity fluctuations in the plastic in the roller gap and the fed-in plastic, as well as unacceptably high temperature variations between the melt in the roller gap and the fed-in plastic. This has a negative effect on the quality of the finished product. Especially disadvantageous is this type of charging in the case of two- and three-roller calenders. Finally, it has been proved that in the above-mentioned charging process, there arise considerable difficulties in the starting of such an apparatus.
It is a further known practice to have an extruder before a calendering or a melt roll machine from which an oscillating conveyor belt leads to the roller mouth. The extruded string is subdivided into small sections, which are conveyed over an oscillating conveyor belt into the roller mouth. There arise similar disadvantages as in the extruder feed system with a swinging arm as described earlier.
Furthermore, it is known to carry out the charging of calendering machines by means of a flat sheet die. The material plasticized by the extruder is drawn off as a film from the flat sheet die and supplied to the roller gap. It is to be regarded as a disadvantage on such a measure that both the thickness, width and throughput of such a charging method are narrowly limited.
A further limit exists in regard to the selection of the usable plastics in which, additionally, the formula is to be maintained very accurately. In the unavoidable color changes there arise complicated cleaning operations. Undesirable also, are the long adjusting times of the apparatus, whereby there results a high loss in material. Further, it was ascertained that in this type of charging, there is an increased tendency for the thermoplastic material to decompose, especially in the border zones of the flat sheet dies. Especially striking is this tendency in the case of rigid PVC. Finally, the width of a die is not adjustable, so that for each film width there has to be available a special sheet die. As a result, the investment costs are increased, and the change-over times are unnecessarily lengthened.
Underlying the present invention, therefore, is the task of eliminating the inadequacies related to the charging apparatus hitherto known and to fulfill the following requirements: the charging of the shearing gap should extend over its entire length, i.e., over the so-called working width. The plastic should have a uniform viscosity in the shearing gap over the entire working width. A constant mean filling height in the shearing gap should be achieved over the entire working width. With increased output capacity, the quality of the finished product should be improved. Further, the charging device should be adaptable in the simplest manner to different working widths. Finally, and this point is of special importance, the number of plastics economically usable for the production of flat plastic structures should be extended.